Stockbyte/Thinkstock(WASHINGTON) -- The announcement that President Obama is planning to step up aid to the Syrian rebels, including sending small weapons and ammunition, is a significant change in the administration's policy on Syria.

But with 90,000 people already killed, a refugee crisis being called the worst in the world, and evidence of Iran and Hezbollah's growing involvement in the crisis, critics question whether the move will be enough to make a difference on the ground.

On Thursday, Sens. John McCain and Lindsey Graham, two of the most outspoken critics of the Obama administration's Syria policy, applauded the decision to provide arms, but called for the U.S. to go further and establish a no-fly zone

"The goal is to end the war. And the only way this war is going to end quickly and on our terms is to neutralize the air assets that Assad enjoys," Graham said on the Senate floor. "We can crater the runways. There are four air bases he uses. We can stop the planes from flying. We can shoot planes down without having one boot on the ground."

McCain and Graham are not the only ones calling for a no-fly zone. Some of America's allies in the region, including Turkey, also have said they would support the action as way to help strengthen the opposition's position on the ground and allow more humanitarian aid to get through.

But the administration maintains a no-fly zone is not the easy solution some are claiming.

"People need to understand that the no-fly zone is not some type of silver bullet that is going to stop a very intense and, in some respect, sectarian conflict, that it's taking place on the ground," National Security Advisor Spokesman Ben Rhodes told reporters on Friday.

So what exactly is a no-fly zone?

A no-fly zone is just what the name indicates; it's an action that stops planes, usually military, from flying in the skies of a designated area. The idea is to keep the military from a rogue nation from using its air power to attack other areas in its own territory or beyond.

No-fly zones are usually authorized by international bodies, like the United Nations or NATO, and the air forces of participating countries are authorized to a nation's air capabilities. They can disable runways at air bases, striking at air defense systems and possibly going as far as shooting down any military aircraft violating the no-fly zone.

The effectiveness of the ban hinges on enforcement, as was the case in Libya, where NATO enforced a no-fly zone though it was the U.S. that bore the brunt of the operations.

It was considered a success that gave the opposition the space and time to take control of the country and overthrow Moammar Gadhafi's regime.

Many critics of the administration's Syria policy have questioned why President Obama is so reluctant to push for a no-fly zone in Syria, as well.

The administration has said Syria is very different from Libya.

"It's dramatically more difficult and dangerous and costly in Syria for a variety of reasons," said Rhodes. "In Libya, you already had a situation where the opposition controlled huge portions of the country and you could essentially protect those portions of the country from the air. You do not have the same types of air defense systems that exist within Syria."

Experts say that Syria's air force is more capable than Libya's and, more importantly, has a complex air defense system that could target military aircraft enforcing a no-fly zone. But that doesn't necessarily preclude putting elements of a no-fly zone in place.

"A no-fly zone is not a monolithic thing," Joseph Holliday, a fellow at the Institute for the Study of War who focuses on the Syria crisis, said at a United States Institute for Peace event last month.

Holliday said that there is a "spectrum" of military options within a no-fly zone, from a full-scale air campaign to finding other creative ways to restrict the Syrian air force's capability. Still, Holliday, who tracks Syria's military capability, warned that the Assad regime seems to be keeping some military fighter jets in reserve specifically to deal with any possible foreign threat.

The Syrian Air Force is "not on its way out," said Holliday, who added that Syria retains "one of the densest air-defense systems in the world."

Administration officials have also questioned whether a no-fly zone would be an effective way to help the opposition defeat Assad and stop the slaughter of civilians.

"In Syria, when you have a situation where regime forces are intermingled with opposition forces and they're fighting, in some instances, block-by-block in cities, that's not a problem you can solve from the air," said Rhodes.

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey told reporters in April that he doubted a no-fly zone could be effective in limiting civilian casualties

"About 10 percent of the casualties that are being imposed on the Syrian opposition are occurring through the use of air power," said Dempsey, who noted that the remaining 90 percent are by direct fire or by artillery. "So, the question then becomes: If you eliminate one capability of a potential adversary, will you be inclined to find yourself in a position to be asked to do more against the rest?"

Dempsey said the United States needs to be careful not to get drawn deeper into the conflict without having clear military objectives. If a no-fly zone was ordered, Dempsey said, the Pentagon would have to factor in the need to knock out Syrian air defenses and develop a search-and-rescue plan for any U.S. fighter pilots that could be downed. Military planners also would have to consider the prospect that Syria might launch retaliatory attacks both within Syria and beyond.

"Now, none of these reasons are reasons not to take action," Dempsey said. "But they all should be considered before we take that first step."